Plastic bottles are widely used containers replacing glass bottles and metallic cans because they are light in weight and high in impact resistance. Plastic bottles used as containers for various kinds of drinks and foods are produced in many cases by blow molding of a thermoplastic resin. As such blow molded containers, bottles (hereinafter abbreviated as “PET bottle”) obtained by blow-molding polyethylene terephthalate (hereinafter abbreviated as “PET”) are widely spread as containers for carbonated drinks, fruit juice drinks, sports drinks, tea drinks, coffee drinks and the like because they are excellent in transparency and glossiness. The PET bottles are generally formed from a single layer of PET.
However, PET is insufficient in gas barrier properties and particularly involves a problem that the gas barrier properties of a body of a PET bottle, the thickness of which is thin, is low. If the gas barrier properties of the PET bottle are low, functions of long-term storage and prevention from deterioration of contents thereof become insufficient. In recent years, the miniaturization of PET bottles has been advanced. A proportion of a bottle surface area to an internal volume becomes high attending on the miniaturization, so that the PET bottles are more and more required to have good gas barrier properties.
On the other hand, attention is paid to a polyglycolic acid (hereinafter may be referred to as “PGA”) as a resin material having high gas barrier properties. Japanese Patent Application Laid-Open No. 10-138371 (hereinafter referred to as “Article 1”) has proposed a gas barrier multilayer hollow container having a layer structure that a polyglycolic acid layer is arranged as a core layer, and thermoplastic polyester resin layers are arranged as inner and outer layers. Example 4 of Article 1 shows a production example of a multilayer hollow container of a 5 layer structure by a co-injection stretch blow molding process, in which a polyglycolic acid layer is provided as a core layer, and inner and outer layers formed of PET layers are each arranged through an adhesive layer. More specifically, Example 4 of Article 1 shows that a multilayer preform having a layer structure of “PET/adhesive/PGA/adhesive/PET” was molded by injection molding, and the multilayer preform was then subjected to stretch blow molding to produce a multilayer hollow container.
Article 1 discloses a wide variety of melt viscosities as to the polyglycolic acid used. However, in each Example, a polyglycolic acid having a melt viscosity of 4,000 Pa·s as measured at a temperature of the melting point+20° C. and a shear rate of 100 sec−1 is used. This melt viscosity, 4,000 Pa·s corresponds to about 1,900 Pa·s as measured at a temperature of 270° C. and a shear rate of 122 sec−1. When the polyglycolic acid having such a high melt viscosity is injection-molded to form a multilayer preform, it is difficult to uniformly control the thickness of the resulting polyglycolic acid layer unless an injection temperature is preset to a high temperature of 255° C. or higher. However, it has been found that when the injection temperature of the polyglycolic acid is preset to a high temperature, the polyglycolic acid remaining in an injection molding machine tends to cause thermal decomposition. When the polyglycolic acid undergoes thermal decomposition, the appearance and gas barrier properties of the resulting multilayer blow molded container are impaired. In addition, according to the co-injection stretch blow molding process disclosed in Article 1, it is difficult to surely embed the polyglycolic acid layer, which is easy to be decomposed under environmental conditions in a thermoplastic resin layer.
Japanese Patent Application Laid-Open No. 2003-20344 (hereinafter referred to as “Article 2”) discloses a stretched multilayer blown container in which a polyglycolic acid having a melt viscosity of not lower than 20 Pa·s, but lower than 500 Pa·s as measured at a temperature of the melting point+20° C. and a shear rate of 100 sec−1 is arranged as a core layer. Example 7 of Article 2 discloses a co-injection multilayer stretch blow molded container of a 5 layer structure that a layer of a polyglycolic acid having a melt viscosity of 45 Pa·s is provided as a core layer, and inner and outer layers formed of PET layers are each arranged through an adhesive layer. However, the co-injection multilayer stretch blow molded container disclosed in Article 2 involves problems that the polyglycolic acid having a low melt viscosity is used and that it is difficult to surely embed the polyglycolic acid layer, which is easy to be decomposed under environmental conditions in a thermoplastic resin layer.
The melt viscosity of not lower than 20 Pa·s, but lower than 500 Pa·s as described in Article 2 corresponds to a melt viscosity of not lower than 9 Pa·s, but lower than 235 Pa·s as measured at a temperature of 270° C. and a shear rate of 122 sec−1. If the molt viscosity of the polyglycolic acid is too low, a hold-over phenomenon that a polyglycolic acid layer forming a core layer is cleaved into plural layers is liable to occur when co-injection molding is conducted together with a thermoplastic polyester resin having a relatively high melt viscosity to form a preform. When the core layer, which will become a gas barrier layer, is cleaved into 2 or more layers, it is difficult to uniformly control the thickness of the core layer.
Japanese Patent Application Laid-Open No. 2003-136657 (hereinafter referred to as “Article 3”) discloses a multilayer container having a layer structure that a polyglycolic acid layer is provided as a core layer, and thermoplastic polyester resin layers are arranged as inner and outer layers. More specifically, Article 3 discloses a process in which a multilayer preform is formed by a co-injection molding process, and the multilayer preform is then subjected to biaxial stretch blow molding to produce a multilayer container. According to the process disclosed in Article 3, a stretched multilayer blow molded container of a form that the polyglycolic acid layer is embedded in a PET layer at the body and bottom of the container is obtained.
Article 3 discloses polyglycolic acids having a wide variety of melt viscosities. However, Examples thereof only disclose a polyglycolic acid having a melt viscosity of 500 Pa·s as measured at a temperature of 240° C. and a shear rate of 100 sec−1. The melt viscosity, 500 Pa·s of the polyglycolic acid described in Article 3 corresponds to 235 Pa·s as measured at a temperature of 270° C. and a shear rate of 122 sec−1. If the melt viscosity of the polyglycolic acid is too low, the hold-over phenomenon that a polyglycolic acid layer forming a core layer is cleaved into plural layers is liable to occur when co-injection molding is conducted together with a thermoplastic polyester resin having a relatively high melt viscosity to form a multilayer preform. When the core layer, which will become a gas barrier layer, is cleaved into 2 or more layers, it is difficult to uniformly control the thickness of the core layer.
Examples 1 and 2 of Article 3 show that a PET having a melt viscosity of 190 Pa·s as measured at a temperature of 280° C. and a shear rate of 100 sec−1 was used as the thermoplastic polyester resin. This melt viscosity of the PET is relatively low. When the thermoplastic resin having a too low melt viscosity is used, the mechanical strength of the resulting multilayer blow molded container is deteriorated.